The City of Palmerston North water
supply comes from Turitea dam and four bores (Keith
Street, Roberts Line, Papaioea Park and Takaro Park).
On a typical day we supply approximately 29,000 m3 of water to
the residents of PN. Of this, typically 60% is supplied from
Turitea with the remaining being supplied from the four bores to
boost the supply during times of peak demand.
The Turitea water source is surface water with the catchment
being the Turitea Reserve. The source of discolouration is
due to the presence of Iron and Manganese in the source water.
The water is piped from the Water Treatment Plant (WTP) to two
storage reservoirs at Ngahere Park. Prior to 2002 town was
fed one trunk main (the 525) from the reservoirs to the Fitzherbert
Bridge, where several 'smaller' mains distribute the water to the
City. In 2002 a second (duplicate) main was commissioned to
boost the supply during peak demand periods (the 600 main was
constructed). The 600 feeds the City from the Ngahere Park
reservoirs to the Fitzherbert Bridge also.
Over the years Iron and Manganese deposits have formed within
the mains and during periods of peak demand (especially during
summer) the increased flow and velocities in the network
effectively scour (pick up) these deposits and distribute them
through the network and ultimately they appear as discoloured
The deposits occur when the residual chlorine in the water
oxidises the Iron and Manganese and small deposits form. The
water is chlorinated to meet the requirements of the New Zealand
Drinking Water Standards.
In order to improve the water quality City Networks commissioned
a project to build the swab
chamber at Centennial Drive Reserve. The purpose of this
was to place a foam swab into the mains at the reservoirs and using
water pressure drive the swab down through the mains and discharge
the resultant discoloured water into the Manawatu River. The
chamber was designed to capture the swab.
Only one main can be swabbed per event. To swab we need to
shut down one main from supply and rely on the other main and the
bore supplies. In order to discharge the discoloured water
from the swabbing chamber into the Manawatu River the PNCC was
required to get a Resource Consent from Horizons Regional
Council. This consent sets certain conditions that must occur
in the Manawatu River before the discharge can occur.
Condition 2 of our resource consent states that "The Permit
shall ensure that the discharge to the Manawatu River only occurs
when the flow in the Manawatu River, as measured at the Teachers
College flow recording station is equal to or greater than 100
cubic metres per second (100m3/s)"
Condition 2A further states that "The Permit shall ensure that
the discharge to the Manawatu River only occurs when the water
turbidity (colouration) of the Manawatu River, as measured at the
Teachers College flow recording station is equal to or greater than
20 Nephelometric Turbidity Units (NTU)"
We are also further restricted by condition 3 which states that
"The permit holder shall ensure that the discharge to the Manawatu
River is restricted to during night time, defined as the time
between one hour after sunset and one hour before sunrise"
These three conditions make it very difficult to predict and
plan for this discharge to occur.